It has been known for a long time that polystyrene beads can be rendered expandable by incorporating in them a blowing agent (or expansion agent) such as a volatile liquid hydrocarbon, for example pentane, and that the beads thus obtained can be used in the manufacture of expanded moulded polystyrene objects. The expanded moulded polystyrene is a relatively rigid and not very dense cellular material, its properties depending mainly on the quantity of blowing agent present in the expandable polystyrene beads and on the cellular structure of the expanded moulded polystyrene.
The expanded moulded polystyrene objects most common in commerce are intended for the heat-insulation of buildings and for the packaging of industrial or food articles. They usually form part of a so-called low-density grade of expanded moulded polystyrene, exhibiting in particular a density of from 10 to 30 g/l approximately. For said type of grade of expanded moulded polystyrene, the quantity of blowing agent present in the expandable polystyrene beads, prior to any expansion, is generally chosen from a range of about from 6 to 9 wt % compared with the total weight of the beads.
The process for manufacturing expanded moulded polystyrene objects generally comprises the following stages: (i) a stage of pre-expansion by heating the expandable polystyrene beads so as to form pre-expanded beads having a desired density, (ii) a stage of stabilisation (“maturing” or “ageing”) of the pre-expanded beads for a specified period and (iii) a stage of moulding of the stabilised beads by heating them in a mould so as to weld the beads to one another.
American U.S. Pat. Nos. 5,086,078, U.S. 5,110,836, U.S. 5,110,837, U.S. 5,114,640, U.S 5,115,066 and U.S. 5,240,657 disclose processes for manufacturing low-density expanded moulded polystyrene objects, generally having a density of from 13 to 32 g/l, particularly from 13 to 18 g/l. Nothing is said about manufacturing medium-density expanded moulded polystyrene objects. The objective of these processes is to reduce emissions into the atmosphere of volatile organic blowing agent when manufacturing low-density expanded moulded polystyrene objects. The processes employ expandable polystyrene compositions in the form of beads containing a specific polystyrene having a narrow polydispersity, in particular of less than 2.0, for example of from 1.7 to 1.98, and exhibiting a high degree of expandability. A blowing agent is incorporated into the polystyrene in an amount of from 2 to 5.5 wt % or from 2 to 4.4 wt %. Furthermore, the expandable polystyrene compositions comprise no plasticising agent. The processes for manufacturing the low-density expanded moulded polystyrene objects comprise a plurality of successive stages of pre-expansion of the beads prior to the moulding stage, for example from 2 to 5 stages, each pre-expansion stage being followed by a stabilisation stage lasting from 1 to 80 hours. Said processes have the disadvantage of employing multiple pre-expansion and stabilisation stages, which make the processes relatively complex, costly and unproductive.
For many years, special grades of expandable polystyrene intended for so-called high- or medium-density expanded moulded polystyrene objects have been developed for specific applications such as the industrial packaging of heavy objects and/or subjected to strong forces, for example to strong pressure, or else been developed for far more specific applications such as helmets or platform shoes.
American U.S. Pat. No. 3,973,884 discloses a process for manufacturing a medium- or high-density polystyrene foam, more particularly with a density ranging for example from 130 to 190 g/l or from 260 to 320 g/l. The process employs expandable polystyrene beads having a content of pentane of from 5.8 to 7.0 wt % or from 5.6 to 6.8 wt %, which content is considered to be slightly less than that of 6.0 to 7.2 wt % applied conventionally for low-density polystyrene foams. It has been observed that in said process the stage of stabilisation of the pre-expanded beads must be carried out over an excessively long period, of the order of one or more weeks.
American U.S. Pat. No. 4,032,609 discloses a process for the pre-expansion and moulding of expandable polystyrene beads, intended for the subsequent manufacture of expanded moulded polystyrene objects in particular of relatively high density, capable of ranging for example from 128 to 240 g/l. The process comprises in particular a pre-expansion of the expandable beads for a specified period at the end of which the beads are subjected to a super-atmospheric pressure by means of a gas, so as to prevent any additional expansion of the beads. Pre-expanded beads of uniform density are thus obtained, which are then moulded into expanded objects more particularly of high density. The expandable polystyrene beads contain a blowing agent in a quantity of from 2 to 15 wt %, more particularly from 3 to 10 wt %. In particular, the American patent shows the use of expandable polystyrene beads containing 6 wt % of pentane which are then intended to be pre-expanded and moulded into high-density objects, for example with a density a density of 192 g/l. It was observed that in this case the stage of stabilisation of the pre-expanded beads must be carried out for an excessively long period more particularly of one or more weeks.
European patent application EP 0 987 293 discloses a process for manufacturing objects of low-density expanded polystyrene which are intended more particularly for packaging. The process is aimed at resolving the problem of the uncontrolled losses to the atmosphere of blowing agent during the transport and the storage of the expandable polystyrene particles, prior to the manufacture of the expanded objects. The process proposes the use of expandable polystyrene particles having a low content of blowing agent, which can be from 0.5 to 4.0 wt % compared with the polystyrene. However, in practice according to the examples, the blowing agent content specifically is equal to 1.6 or to 2.1 wt %. The process comprises a stage of pre-expansion of the expandable polystyrene particles so as to form “porous” (or pre-foamed) particles having a residual content of blowing agent less than or equal to 2 wt % and a very high bulk density of 600 to 200 g/l. It is emphasised that, contrary to the conventional processes, the “porous” particles are for practical purposes not expandable any further. The process therefore necessarily comprises an additional stage of impregnation of the “porous” particles by means of a fresh quantity of blowing agent, so as to render said particles expandable once again and to then mould them into expanded finished objects. However, the process has the drawback of employing an additional stage of impregnation by a blowing agent after the pre-expansion stage. Moreover, the patent application says nothing about a possible stage of stabilisation of the pre-expanded particles and the operating conditions of said stage. In addition, it is silent about manufacturing medium-density expanded moulded polystyrene objects.
International patent application WO 02/46284 discloses a process for manufacturing expanded moulded polystyrene objects in particular of low density, which can be for example from 12 to 30 g/l. The process proposes reducing the losses to the atmosphere of blowing agent, such as pentane, in particular during the stages of expansion and moulding of the expanded objects. The process employs an expandable polystyrene composition in the form of particles or pellets, having a content by weight of blowing agent of less than 10.0%, preferably less than 9.0%, and in particular of between 3.0 and 9.0%. Furthermore, the expandable polystyrene composition comprises no plasticising agent. The process comprises a stage of pre-foaming by heating (between 70° C. and 110° C.) of the expandable polystyrene particles, under an absolute pressure of 70 to 170 kPa, so as to form “foamed cellular particles” having a very high bulk density in particular of between 550 and 200 g/l, and a content by weight of blowing agent of less than 6%, in particular of between 2.0 and 5.0%. The advantage of said process compared with that described in European patent application EP 0 987 293 comes from the fact that the “foamed cellular particles” have a sufficient quantity of blowing agent such that said particles are still expandable and no longer require to be impregnated by a fresh quantity of blowing agent. The process for manufacturing expanded moulded polystyrene objects comprises in addition the other conventional stages, that is to say pre-expansion, stabilisation and moulding. However, the result is that the pre-foaming stage constitutes an additional stage compared with the conventional processes, which makes the process complex, costly and unproductive.
French patent application FR 2 780 406 discloses an expandable polystyrene composition in the form of beads comprising from 2 to 9 wt % of a blowing agent compared with the polystyrene. However, in practice according to the examples, the blowing agent content effectively is substantially high and specifically in the range of from 4.4 to 6.28 wt %. Furthermore, the expandable beads are not used for manufacturing medium-density expanded moulded polystyrene objects, but only objects of high density, e.g. of from 13.3 to 26 g/l according to the examples.